1. Field of the Invention
The present invention relates to a calibration jig for an optical tomographic imaging apparatus and a method for generating a calibration conversion table. The present invention particularly relates to a calibration jig which is used for calibrating a probe optical tomographic imaging apparatus with the calibration jig being attached to an attachment section for receiving an optical probe of the optical tomographic imaging apparatus, and a method for generating a calibration conversion table.
2. Description of the Related Art
Optical tomographic imaging apparatuses, which use OCT (Optical Coherence Tomography) measurement, have sometimes been used to acquire tomographic images in body cavity. The optical tomographic imaging apparatus acquires an optical tomographic image of a subject to be measured as follows. First, wideband light emitted from a light source is divided into measurement light and reference light with an optical interferometer. Then, the measurement light is guided through an optical axis scanning means to be applied to the subject to be measured, and the optical axis scanning means scans the subject to be measured with the measurement light in one dimensional or two dimensional direction which is perpendicular to the optical axis. Then, the light reflected from the subject to be measured returns to the interferometer, where the reflected light is combined with the reference light to provide interference light between the reflected light and the reference light. Then, the optical tomographic image is generated based on intensity of the interference light. There are roughly two types of optical axis scanning means: a space scanning system that uses a galvanic mirror or polygon mirror to effect linear scanning with the light propagating in space; and a probe system that guide the light to propagate through an optical fiber and rotates the output end of the optical fiber to effect radial scanning. A typical example of the space scanning system is a fundus OCT apparatus for observing the fundus. Examples of the probe system include a vascular OCT apparatus for observing vascular wall using an optical fiber guided through a vascular catheter, and an endoscopic OCT apparatus which is combined with an endoscope to observe a wall of digestive tract, etc. Examples of the technique used to achieve optical scanning with the probe system include, besides the radial scanning, moving the distal end of the probe to effect linear scanning, or providing a small scanning mechanism in the probe to effect linear scanning.
OCT measurement techniques are roughly classified into TD (Time Domain)-OCT measurement techniques and FD (Fourier Domain)-OCT measurement techniques. Recently, the FD-OCT measurement techniques are attracting attention since they allow high speed measurement. Typical optical tomographic imaging apparatuses that carry out the FD-OCT measurement include an SD (Spectral Domain)-OCT system and an SS (Swept Source)-OCT system.
The SD-OCT system uses wideband low-coherent light, and decomposes the interference light into optical frequency components with a spectral means. Then, intensity of the interference light corresponding to each optical frequency component is measured using a photodetector array, or the like, and thus obtained interference signal is subjected to Fourier transform analysis on a computer, thereby acquiring information of reflectance in the depth direction, i.e., tomographic information.
The SS-OCT system uses, as a light source, a laser with optical frequency thereof swept with time, and measures temporal characteristics of the intensity of the interference light corresponding to temporal change of the frequency of the interference light. Then, thus obtained interference signal is subjected to Fourier transform on a computer, thereby acquiring information of reflectance in the depth direction, i.e., tomographic information.
In the FD-OCT, in order to obtain the tomographic image, i.e., positional information in the depth direction, the interference signal, which has been obtained at equal intervals with respect to a wavenumber k of the measurement light, is subjected to the Fourier transform. However, actual data obtained in the SD-OCT system is obtained at equal intervals with respect to spatial displacement of spectral wavelengths, and actual data obtained in the SS-OCT apparatus is obtained at equal intervals with respect to time. This is because that the relationship between the wavenumber k and the spatial displacement or time is nonlinear due to characteristics of the light source used, influence of components of the apparatus, etc., and thus the Fourier-transformed signal has a distorted waveform. Therefore, when the spatial displacement axis or the time axis is converted into the wavenumber axis, it is necessary to correct for the distortion to achieve correct conversion. An operation of generating a conversion table (calibration conversion table) to be used for this conversion is called calibration.
To achieve the calibration for the SS-OCT system, two types of methods have been proposed. One is a calibration method that corrects only frequency sweep characteristics of the light source, which has the greatest influence. In this method, a part of the measurement light is passed through an interference filter or fiber Bragg grating, and the transmitted light and the reflected light are measured with light receiving elements. Then, a time t-to-wavenumber k characteristic of the light source is calculated from the thus obtained optical signal to generate the calibration conversion table. This method, however, has drawbacks such that an expensive instrument is necessary for evaluation, it allows only correction for distortion of the frequency sweep characteristics of the light source, and influence of dispersion at the interferometer or the probe, for example, cannot be corrected for. The other is a calibration method proposed in Japanese Unexamined Patent Publication No. 2007-101365, which achieves the calibration by using the optical tomographic imaging apparatus itself without using a special instrument as in the method described above. Specifically, the frequency-swept light source is monitored with the optical tomographic imaging apparatus to detect a spectral interference signal as a time signal. Then, time dependency of the sweep frequency is found from the spectral interference signal to calibrate time-dependent characteristics of the sweep frequency of the frequency-swept light source.
On the other hand, for calibrating the SD-OCT system, wavelength of the light is spatially dispersed (decomposed) with a grating and is measured with a detector array. The spatial dispersion at the grating is linear with respect to a wavelength λ, i.e., the inverse of the wavenumber k. Therefore, in the case of the SD-OCT, influence of the effect of the spatial dispersion at the grating being nonlinear with respect to the wavenumber k is the greatest. In order to correct for only this influence, wavelength dispersion at the grating is theoretically found to generate the calibration conversion table. Alternatively, it is easily conceived to use a spectrometer to measure the wavelength with respect to the spatial displacement to generate the calibration conversion table. In this method, however, influence of the dispersion at the interferometer or probe, for example, cannot be corrected. The dispersion here means change of optical path difference depending on wavelength due to wavelength dependency of refraction index in a medium. The above-mentioned Japanese Unexamined Patent Publication No. 2007-101365 also discloses that a method similar to the above-described method can be applied to the SD-OCT.
The above-mentioned Japanese Unexamined Patent Publication No. 2007-101365, however, does not disclose specific means in a case where the above-described calibration method is used. In a conventional basic OCT experimental apparatus, a measuring device combining a microscope and an optical axis scanning mechanism is placed on a laboratory table, and measurement is carried out with placing a measured sample on the laboratory table. In this view, it is easily conceived that, as a specific means in the case where the above-described calibration method is used, the device placed on the laboratory table may be used in stead of an optical probe, and a mirror reflector may be placed in stead of the measured sample. In this method, however, wavelength distortion characteristics that are different from the characteristics in a case where an optical probe is used are introduced due to influence of optical components, such as a lens, of the measuring device, and this makes the calibration incomplete. Further, ease and convenience are lost due to the size of the apparatus.
The reason is as follows. Unlike the measuring device combining a microscope and an optical axis scanning mechanism placed on the laboratory table, an optical probe used in actual measurement has a small diameter and is long from the nature of the use thereof. Further, since a protective member (sheath) for protecting the optical probe from a living body completely separate the optical axis scanning mechanism from the operator, it is very difficult to hold, fix and finely adjust the optical axis. Therefore, when the optical probe covered with the sheath, which is used in actual measurement, is attached to the attachment section, to which the optical probe is optically coupled in a removable manner, of the optical tomographic imaging apparatus and a some type of reflective member is placed in the vicinity of the distal end of the optical probe to apply the measurement light to the reflective member, it is difficult for the operator to make the reflected light being reflected from the reflective member enter the optical probe with accuracy, and thus the calibration may not be repeatable. In addition, since optical probes used in actual measurement have manufacturing variations, it is more difficult to achieve repeatable calibration if different optical probes are used in calibration operations carried out at different times.
Further, in order to obtain accurate tomographic information of the subject to be measured using the calibrated optical tomographic imaging apparatus, it is desirable to calibrate the optical tomographic imaging apparatus in substantially the same state as the actual state of measurement which is carried out using the optical probe attached to the apparatus. In particular, from the nature of the use, the optical probe is covered with a sheath that transmits light, so that the measurement light from the optical probe transmits through the sheath to be applied to the subject to be measured, and the reflected light from the subject to be measured transmits through the sheath again to enter the optical probe. When the measurement light and the reflected light transmit through the sheath, the measurement light and the reflected light are dispersed due to the wavelength dependency of refraction index, and this distorts the interference signal. Therefore, in order to provide accurate tomographic information of the subject to be measured, it is necessary to correct for the distortion due to the dispersion at the sheath.
Further, the method for generating a calibration conversion table disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 2007-101365 involves cutting out a peak of interest from a plurality of spectral peaks (FIG. 11A) after the interference signal has been Fourier transformed. In general, criteria for selecting a cut-out region A for cutting out the peak largely vary depending on the technique used and conditions, and it is difficult to cut out the peak in a uniform manner. Therefore, this method is time consuming and does not provide high accuracy.